In next generation multimedia mobile communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and radio data) at a higher data rate in addition to the early-stage voice service. Further, a 3rd generation wireless communication system is followed by a 4th generation wireless communication which is currently being developed aiming at support of a high-speed data service of 1 gigabits per second (Gbps) in downlink and 500 megabits per second (Mbps) in uplink. Furthermore, the wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as a fading phenomenon caused by a path loss, noise, and multipath, an inter-symbol interference (ISI), a Doppler effect caused by mobility of a mobile station (MS), etc. Various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
Orthogonal frequency division multiplexing (OFDM), multiple input multiple output (MIMO), etc., are techniques for supporting reliable high-speed data services.
An OFDM system capable of reducing an ISI effect with a low complexity is taken into consideration as one of post-3rd generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on N separate subcarriers. The subcarriers maintain orthogonality in a frequency dimension.
A MIMO technique improves data transmission/reception efficiency by using multiple transmit (Tx) antennas and multiple receive (Rx) antennas. Examples of the MIMO technique include spatial multiplexing, transmit diversity, beamforming, etc. A MIMO channel matrix depending on the number of Rx antennas and the number of Tx antennas can be decomposed into a plurality of independent channels. Each independent channel is referred to as a layer or a stream. The number of layers is referred to as a rank.
In order to restore data transmitted from a transmitter, a receiver needs to perform channel estimation. The channel estimation is defined as a process in which a distorted signal is restored by compensating for signal distortion when fading occurs due to rapid environmental changes. In general, the channel estimation requires a pilot known to both the transmitter and the receiver. Since the MIMO system experiences a channel corresponding to each antenna, there is a need to arrange pilots by considering multiple antennas. Therefore, a pilot overhead significantly increases in a pilot structure for supporting MIMO. The pilot overhead can be defined by a ratio of the number of subcarriers allocated to a pilot to the total number of subcarriers. When the pilot overhead is great, there is a problem of the decrease in the number of data subcarriers used for data transmission. This results in the decrease in a data throughput and the decrease in spectral efficiency. As a result, overall system capability may deteriorate.
However, a system supporting MIMO can use either some of a plurality of antenna resources or only one antenna resource. Examples of a case where pilot resources cannot be sufficiently utilized include a case where an MS existing in a location causing the most significant interference to a neighboring cell transmits a signal to the neighboring cell with a different MIMO configuration, a case where a spatial multiplexing or a diversity gain cannot be attained due to a high correlation between antenna channels, a case where MSs having different antenna configurations are scheduled by grouping them in a virtual MIMO manner, a case where a pilot structure of a macro cell is used when a channel property is excellent in a small cell such as a femto cell, etc. In this case, there may be a problem in that resources allocated to the pilots cannot be entirely utilized in a pilot structure having a great overhead.
Accordingly, there is a need for a data transmission method capable of effectively utilizing radio resources allocated to pilots when using a pilot structure supporting multiple antennas.